A band pass filter constituted by a micro strip line structure is illustrated i FIG. 1 and a cross-sectional view of that filter is illustrated in FIG. 2. The band pass filter of FIG. 1 comprises a dielectric plate 1 made of, for example, polyolefines, teflon or ceramics, an input conductor 21, an output conductor 22, ring conductors (closed end conductors) 51 and 52, a coupling conductor 6 and a ground conductor 7. The input conductor 21, the output conductor 22, the ring conductors 51, 52 and a coupling conductor 6 are made of, for example, copper and are formed by an etching process on the top surface of the dielectric plate 1. The ground conductor 7 is made of, for example, copper and is attached on the bottom surface of the dielectric plate 1.
A signal supplied to the band pass filter of FIG. 1 is transmitted through the input conductor 21, the ring conductor 51, the coupling conductor 6, the ring conductor 52 and the output conductor 22. The frequency selection is carried out due to the resonance characteristics of the ring conductors 51 and 52. A band pass filtering characteristic of the stagger type is obtained in the band pass filter of FIG. 1, in which a loss characteristic on the order of 5 to 6 dB at 3 GHz and a 3 dB band-width characteristic of approximately 12 to 15 MHz are attained.
The close proximity of the input conductor 21 and the output conductor 22, between which a narrow gap 3 is formed, gives the band pass filter of FIG. 1 a trap characteristic in the frequency ranges just above and just below the pass band.
The attenuation characteristics of the band pass filter of FIG. 1 is illustrated in FIG. 3. In FIG. 3, the abscissa represents the frequency f in MHz and the ordinate represents the attenuation ATT in dB. The frequency f.sub.0 is the central value, for example 3 GHz, of the pass band frequencies. Trap regions are formed corresponding to the frequencies f.sub.1 and f.sub.2. The frequencies f.sub.1 and f.sub.2 are symmetrical with respect to the frequency f.sub.0. These trap regions are formed due to the close location of the input conductor 21 and the output conductor 22. The narrower the gap 3 between the input conductor 21 and the output conductor 22, the less the difference between trap frequencies f.sub.1, f.sub.2 and the central pass band frequency f.sub.0. Thus, in the band pass filter of FIG. 1, a narrow band pass characteristic with a single peak stagger resonance feature is obtained.
However, the band pass filter of FIG. 1 is disadvantageous because of the facts that: the pattern of the ring conductors 51, 52 is relatively large and complicated; the existence of the coupling conductor 6 is necessary; the adjustment of the coupling between the ring conductors 51, 52 and the coupling conductor 6 is necessary, and; accordingly, the size of the band pass filter is caused to be large and the design of the structure of the band pass filter tends to become complicated.
The present invention is directed to prevent the above described disadvantages encountered in the band pass filter described above. This band pass filter is described in Japanese Utility Model application No. 54-10382, assigned to the same assignee of the present invention, this application having been laid open in Japan on Aug. 9, 1980.
An example of a prior art band pass filter is disclosed in the following publication.
(1) Japanese Utility Model Application Laid-open No. 54-71940